1. Field of the Invention
The present invention relates to a silicon nitride based sintered body having excellent mechanical properties in middle and low temperature ranges from room temperature to 1100.degree. C. as structural ceramic materials to be used for automobile parts, wear-resistant tools, and the like. The invention also relates to methods for producing the same.
2. Description of the Prior Art
Silicon nitride is a material well balanced in strength, fracture toughness, corrosion resistance, wear resistance, thermal shock resistance, oxidation resistance, and the like, so that it is used over a wide range of fields such as cutting tools and engine parts. In particular, it has recently been drawing industrial attention as a structural material of automobile engines, gas turbines, and the like. However, to apply silicon nitride based sintered bodies to those fields which demand high reliability to materials such as in automobile engines, it is essential to further improve their fracture toughness so as to overcome their brittleness, and to improve their strength at the same time.
For example, as disclosed in Japanese Patent Laid-Open Publication No. SHO 61-91065 and Japanese Patent Laid-Open Publication No. HEI 2-44066, there have been proposed silicon nitride single-phase materials that show improved mechanical characteristics such as strength by generating a combination of crystal phases, or a composite crystal phase, of equiaxial particulate .alpha.'-sialon (general formula: M.sub.x (Si, Al).sub.12 (O, N).sub.16, wherein M represents Mg, Ca, Li, and rare-earth elements) and columnar .beta.'-sialon (including .beta. type silicon nitrides). However, as apparent from their embodiments, the methods for producing a sintered body that show a strength characteristic of their resulting sintered bodies stably exceeding 100 kg/mm.sup.2 by flexural strength are in all cases by the hot press process, such that high strength characteristic has not yet been attained stably from the industrial point of view.
Also, there have been proposed composite materials, such as disclosed in Japanese Patent Laid-Open Publication No. HEI 4-202059, in which 1 to 500 nm fine particles are dispersed in silicon nitride and sialon having a longer-axis diameter of 0.05 to 3 .mu.m and an aspect ratio of 3 to 20. The composite materials indeed show a strength of 167 kg/mm.sup.2 at a maximum in the embodiments, but may incur deterioration in strength because they may contain coarse silicon nitride. Further, the composite materials only have a Weibull coefficient of 9 or so. Thus, high strength characteristic could not be attained with stability, also in this case.
Yet further, as disclosed in Japanese Patent Laid-Open Publication No. HEI 4-295056, there has been proposed a sintered body in which different types of particles are dispersed in a grain boundary phase of silicon nitride having a columnar structure. This sintered body has maximum particle sizes as much as 2 .mu.m to 3.5 .mu.m in shorter-axis diameter and 10 .mu.m to 14 .mu.m in longer-axis diameter, such that the matrix itself can make a fracture source. The strength in its embodiment is 158 kg/mm.sup.2 at a maximum, and the baking temperature is above 1800.degree. C. Thus, the sintered body could not be sufficient in terms of productivity and cost.
in the prior art attempts to improve the strength and toughness of silicon nitride based sintered bodies, as described above, it has been very difficult to improve the strength and the fracture toughness at the same time, due to the fact that enhancing the strength by forming the fine grained structure would result in lowered fracture toughness and, conversely, enhancing the fracture toughness by presenting columnar crystals through addition of whiskers or grain growth of silicon nitride would incur deterioration in strength.